1. Field of the Invention
The invention relates to sulfur removal from gas feeds for sulfur sensitive syngas generation and hydrocarbon synthesis. More particularly, the invention relates to producing a very low sulfur gas feed for syngas generation, by sequentially contacting the feed with zinc oxide and then nickel to reduce the sulfur content to less than 0.1 vppm and preferably less than 80 vppb. Contacting the syngas with zinc oxide downstream of the syngas generator reduces the total sulfur in the syngas to the less than 10 vppb which is needed for hydrocarbon synthesis.
2. Background of the Invention
Catalytic processes which employ a highly sulfur sensitive catalyst are well known and include, for example, (i) catalytic steam reforming and synthesis gas (syngas) generation in which a light hydrocarbon gas feed is steam reformed in the presence of a steam reforming catalyst and (ii) hydrocarbon synthesis in which a syngas mixture of H.sub.2 and CO is reacted to form liquid hydrocarbons in the presence of a suitable Fischer-Tropsch catalyst. Such catalysts include, for example, one or more supported Group VIII catalytic metal components, with nickel being preferred for steam reforming due to it's low cost compared to noble metals and cobalt for some hydrocarbon synthesis (HCS) processes. These catalysts are sensitive to sulfur poisoning which can cause rapid loss of production and require frequent catalyst addition and replacement. Certain HCS catalysts are highly sensitive to sulfur poisoning. Other process which employ a sulfur sensitive catalyst include water gas shift and hydroisomerization. Steam reforming converts a gas mixture of a light hydrocarbon and steam to a mixture of H.sub.2 and CO, and some unreacted CH.sub.4. It is also used in syngas processes which form a syngas comprising a mixture of H.sub.2 and CO as an HCS feed, as a reducing gas and as a source of hydrogen. Syngas processes include catalytic and non-catalytic partial oxidation, steam reforming and a combination of partial oxidation and steam reforming. In the latter case, a feed comprising a mixture of a hydrocarbon, preferably light alkane (e.g., C.sub.1 -C.sub.4) and more preferably primarily methane, as in natural gas, along with oxygen, and steam are fed into a syngas generator (reactor) in which the alkane is partially oxidized and catalytically steam reformed in the presence of a supported nickel or noble metal catalyst. The catalyst can be in either a fixed or a fluidized bed, with the latter known for its excellent mixing and heat transfer characteristics. The fixed bed process is known as an autothermal process and the fluid bed syngas generating (FBSG) process is well known and is disclosed, for example, in the literature and in U.S. Pat. Nos. 4,888,131 and 5,160,456. Sulfur in the hydrocarbon feed deactivates the steam reforming catalyst, thereby reducing productivity and requiring frequent additions of fresh catalyst. This is much more pronounced in FBSG than in a fixed bed syngas process. Sulfur removal using zinc oxide and zinc oxide mixed with nickel or nickel oxide for reducing the sulfur content of a gas is known and disclosed, for example, in U.S. Pat. Nos. 3,441,370 and 5,244,641. U.S. Pat. No. 5,114,689 discloses sulfur removal employing a sequence of a regenerable molecular sieve physical adsorbent, followed by a zinc oxide chemical absorbent, to produce a gas containing from 1-10 vppm sulfur, with about 5 vppm the actual target. However, it has recently been found that as little as 0.1 vppm of sulfur in the feed gas to a FBSG results is loss of catalytic activity. Further, it is desired to reduce the total sulfur in syngas feed to an HCS reactor to less than 10 vppb. Consequently, there is a need for a sulfur removal process which will reduce the sulfur level in the syngas feed to less than 0.1 vppm and to less than 10 vppb in the syngas.